The gain control of an open-loop amplifier (small signal or large signal) is usually done by varying one or more of the gain parameters e.g. Gm (transconductance of the transistor), magnitude of source or load impedance etc, thus achieving a range of gain values.
Amplifier circuits realized using physical devices available in an integrated circuit process (e.g. transistors, resistors, capacitors etc) are subject to uncontrollable statistical variations in process, supply voltage and temperature (PVT). Therefore amplifier gain can drift from the desired gain value due to PVT variations. This problem may be alleviated in DC and low-frequency amplifiers by employing the technique of negative feedback; here open-loop gain of the amplifier is traded off to reduce the effects of PVT variations.
The application of similar techniques for gain control is usually not affordable in RF amplifiers because of scarcity of available gain at high frequencies within reasonable power consumption. RF amplifiers therefore tend to be operated in open-loop fashion that makes them susceptible to change in their performance (e.g. Gain) due to PVT variations.
Some prior art documents include U.S. Pat. No. 5,768,694 and U.S. Pat. No. 7,065,155.
U.S. Pat. No. 5,768,694 discloses an AGC circuit used in radio transmitters for satellite communications and the like intended to eliminate non-linearity compensators for detectors and reduce the radio transmitters in size and complexity. An RF input signal is amplified and output while passing through input splitting means, programmable attenuator, amplifier, and output splitting means. At the same time, a part of the RF input signal branches off at the input splitting means to input detecting means, where it is detected so as to obtain an input signal amplitude. In addition, a part of the amplifier output is split off at the output splitting means, then attenuated by attenuation means and finally reaches output detecting means, where the signal is detected so as to obtain an output signal amplitude after attenuation. Control means adjusts the attenuation factor of the programmable attenuator so that the input of the input detecting means will be equal to the input of the output detecting means.
U.S. Pat. No. 7,065,155 discloses a transceiver and a method of operating the same that includes in the transmitter a power control circuit that operates on an analog differential signal containing data packets individually. The power control circuit initially transmits a series of data symbols with known values, periodically strobes the transceiver system for correct power levels and incrementally increases the power level of the transceiver until the optimal gain is reached, without exceeding the maximum output power.
Other prior art documents include European publications EP 0473299 and EP 424009; GB published application 2161335, and U.S. Pat. No. 6,091,942.